Field of the Invention
The present invention relates to radioactive gas measurement apparatuses. In particular, the present invention relates to a positron annihilation gamma ray suppression type radioactive gas measurement apparatus (hereinafter, also referred to as a “radioactive gas measurement apparatus”) and failed-fuel detection system which measure the radiation generated from a nuclide to be measured by significantly suppressing the influence of an annihilation gamma ray from a radioactive gas containing the nuclide to be measured and a positron emitter nuclide.
Description of the Related Art
Conventionally, in a nuclear power plant, the amount of a fission product contained in a primary coolant (nuclear reactor water) or a main steam is always monitored for the purpose of confirming the soundness of a nuclear reactor fuel. In particular, the nuclear reactor water or main steam often contains a lot of N-13 (nitrogen-13). For this reason, in the measurement of an index nuclide (I-131 (iodine-131) or Xe-133 (xenon 133)) for monitoring the soundness of this failed fuel, a measurement technique for suppressing the influence of a background radiation caused by an annihilation gamma ray which N-13 radiates is indispensable.
Conventional radioactive gas measurement apparatuses and failed-fuel detection systems are described. JP-A-7-218638 discloses a failed-fuel detection system which detects the I-131 concentration in nuclear reactor water using a prior art measurement method for suppressing positron annihilation gamma rays. In the apparatus described in JP-A-7-218638, the influence of an annihilation gamma ray emitter nuclide contained in nuclear reactor water is reduced by performing an anticoincidence processing on the positron annihilation gamma ray with the use of two radiation detectors, and then 1-131 which is the index for failed fuel detection is measured.
In addition, JP-A-2001-235546 discloses a radioactive gas measurement apparatus and a failed-fuel detection system which detect the radiation concentration in a radioactive gas using a measurement method for suppressing positron annihilation gamma rays. In the apparatus described in JP-A-2001-235546, with respect to a radioactive gas, the influence of an annihilation gamma ray emitter nuclide contained in a radioactive gas is reduced by performing an anticoincidence processing on the positron annihilation gamma ray with the use of two radiation detectors, and then Xe-133 which is the index for failed fuel detection is measured.
FIG. 17 shows one example of the configuration of a conventional radioactive gas measurement apparatus including an anticoincidence circuit. In the conventional radioactive gas measurement apparatus shown in FIG. 17, a radioactive gas measurement cell 42 is provided in inlet/outlet pipes 40a, 40b for radioactive gas, which primarily contains a bleed gas in a reactor condenser system and is referred to as an off-gas, and an anticoincidence processing is performed using two detectors, i.e., a main detector 43 and a sub-detector 44, provided within a shield 41, to reduce the influence of an annihilation gamma ray emitter nuclide (N-13) contained in the gas, then Xe-133 which is the index for failed fuel detection is effectively measured.
These measurement apparatuses are intended to detect whether or not there is a failed fuel by reducing the influence of annihilation gamma rays by about 50% to 20% and examining the index nuclide, in the measurement of the index nuclide (I-131, Xe-133) for failed fuel detection during the normal operation of a nuclear reactor.
JP-A-2005-9890 discloses a conventional radioactive gas measurement apparatus including a mechanism to automatically vary the position of a collimator. The gas radioactive concentration measurement apparatus described in JP-A-2005-9890 comprises a collimator capable of automatically varying its position between a radioactive gas measuring vessel (measurement cell) and a radiation detector, thus enabling the radiation measurement corresponding to a range from a low level to a high level of the radioactive gas concentration to be measured in the measurement cell.
Furthermore, JP-A-2001-141829 discloses a conventional distribution measuring method for measuring a radioactivity distribution in a radioactive fluid in a pipe and a radioactivity distribution in an inner wall of the pipe using a radioactivity measurement apparatus which includes a measurement system, wherein a collimator and a radiation detector integrally move, in the pipe through which the radioactive fluid flows. In the method described in JP-A-2001-141829, the amount of radioactivity stuck to the pipe inner wall area and the amount of radioactivity of the fluid to be measured are determined by calculations from the measured values at two or more places where a measurement range of the pipe inner wall area and a measurement range of the volume of the fluid to be measured in the pipe differ, respectively, along the pipe through which the fluid to be measured flows.